TUTORIAL – AIR CONDITIONING (CHAPTER 7 – MEC551)

1) Airstream at 25°C, 35% relative humidity at rate 25 m3/min is mixed with another

airstream at 15°C, 90% relative humidity at rate 30 m3/min. Assuming that the

mixing process occurs steadily and adiabatically at a pressure of 120 Kpa,

determine;

i) the specific humidity, ω3,

ii) the relative humidity, Ø3

iii) the enthalpy h3 and

iv) the exit temperature, T3.

(20 marks)

APRIL 2009

2) Mr. Tony Adam would like to design an air conditioning system that contains a

humidifying section after a heating section. He has listed the following

requirements for the system:

i) The total pressure of the system is 110KPa.

ii) Saturated steam vapor at 100°C is supplied through the humidifier.

The temperature at the humidifying section is 30°C and 70% relative humidity.

The air enters the heating section at 15°C, 80% relative humidity and at rate 0.7

m3/s. The temperature after the heating section should be 25°C. Could you help

Mr. Adam to sketch the system diagram and determine:

a) the amount of heat that is added through the heater in (KJ/s),

b) the relative humidity after the heating section, and

c) the rate of water is added to the air in the humidifying section.

(20 marks)

OCTOBER 2009

Atmospheric air at 10°C and 40 percent relative humidity flows at a rate of 20 m3/min. It

is first humidified by the injection of steam to 20°C and 80 percent relative humidity. The

air is then passed through a 3.15 kW heating coil. Assuming the entire processes take

place at 100 kPa, determine:

i) the mass flow rate of steam required at the humidifying section, and

ii) the final temperature of the air.

(20 marks)

APRIL 2010

A stream of return air at 40°C and 30 percent relative humidity is mixed adiabatically with a stream of fresh air intake. The mass flow rates of the two streams are the same. A single air stream exits at 30°C and 40 percent relative humidity with a volume flow rate of 60 m3/min. If the mixing process occurs at 1 atm, determine:

a) the mass flow rate of the fresh air (kg/s),b) the volume flow rate of the return air (m3/min), andc) the temperature of the fresh air (°C).

(20 marks)

OCTOBER 2010